If the same is true in humans, it may be futile to try to catch up on missed sleep, say US scientists.

They think it may one day be possible to develop a drug to protect the brain from the side-effects of lost sleep.

The study, published in The Journal of Neuroscience, looked at lab mice that were kept awake to replicate the kind of sleep loss common in modern life, through night shifts or long hours in the office.

We now have evidence that sleep loss can lead to irreversible injuryProf Sigrid Veasey, University of Pennsylvania School of Medicine

A team at the University of Pennsylvania School of Medicine studied certain brain cells which are involved in keeping the brain alert.

After several days of sleep patterns similar to those followed by night workers - three days of night shifts with only four to five hours sleep in 24 hours - the mice lost 25% of the brain cells, in part of the brain stem.

The researchers say this is the first evidence that sleep loss can lead to a loss of brain cells.

But they add that more work needs to be done to find out if people who miss out on sleep might also be at risk of permanent damage.

Prof Sigrid Veasey of the Center for Sleep and Circadian Neurobiology, told BBC News: "We now have evidence that sleep loss can lead to irreversible injury.

"This might be in a simple animal but this suggests to us that we are going to have to look very carefully in humans."

She said the next step was to examine the brains of shift workers after death for evidence of any loss of brain cells.